The prior-art example of the beam index color cathode ray tube is shown in FIG. 3. In this figure, the cathode ray tube is comprised of a screen panel 11 in the form of a flat plate, a funnel part 12 and an electron gun 13. On the inner surface of the screen panel 11 there are coated a large number of parallel vertically extending color phosphor stripes 22. These stripes 22 are in the form of triplets of red, green and blue phosphor stripes and arranged in the order of red phosphor stripes 22R, green phosphor stripes 22G and blue phosphor stripes 22B cyclically as indicated by the arrow mark H. A large number of guard bands 23 of, for example, carbon black, are provided between adjoining ones of these color phosphor stripes. A metal back 24 formed of a layer of electrically conductive metal such as aluminium is applied on the overall surfaces of the color phosphor stripes 22 and the guard bands. On the metal back 24, there are applied a plurality of index signal detecting phosphor stripes or so-called index stripes 25 at positions corresponding to preset ones of the guard bands 23. On a portion of, for example, the funnel part 12, there are provided a plurality of photo-sensors or light receiving elements 14 for sensing the light beam or electro magnetic waves emitted upon impingement of the electron beam on these index stripes 25.
In the color cathode ray tube showing in FIG. 3, the electron beam 15 emitted from the electron gun 13 impinges on the tricolor phosphor stripes 22R, 22G, 22B for exciting and emitting the light from these stripes. It also impinges on the index stripes 25 for exciting and emitting the index light (or electro-magnetic waves) 16 from these stripes. The light 16 is sensed by the photo-sensors 14 for deriving the index signals. The switching signals for switching the respective prime color signals (red, green and blue signals) of the video color signal or so-called color switching signals, are evolved on the basis of these index signals.
It will be noted that, in the instances wherein the color switching signals are evolved on the basis of these index signals, the relation of a so-called non-integral system is preferably, maintained between the frequency f.sub.I of the index signals and the frequency f.sub.S of the color switching signals such that ##EQU1##
Wherein m and n are natural numbers that are relatively prime. It is because the color signals are adversely affected by the index signals in the integral system, that is, the system wherein the frequency f.sub.I is equal to some whole or integral number multiple of the frequency f.sub.S. The relative disposition between the color phosphor stripes 22R, 22G and 22B and the index stripes 25 for the typical case of the nonintegral system wherein m=3 and n=2, is shown in FIG. 4.
In this figure, three index stripes 25 are associated with two triplets on sets of red (R), green (G) and blue (B) color phosphor stripes 22R, 22G and 22B. The widthwise distance between the centers of any adjoining pair of the adjoining color phosphor stripes 22 or the pitch of the color phosphor stripes 22 is constant, whereas the widths W.sub.R, W.sub.G and W.sub.B of the respective color phosphor stripes 22R, 22G and 22B are also equal to one another (W.sub.R =W.sub.G =W.sub.B). Hence, the widths of the guard bands 23 are also constant and equal to one another. The index stripes 25 of the constant width W.sub.I are centered on these guard bands 23 so as to satisfy the non-integral relation, that is, at the rate of three index stripes to two triplets described above. These index stripes 25 are arranged with the phase difference of p/2 with respect to the constant-pitch color phosphor stripes 22, and are arranged at a constant pitch equal to 2p.
FIG. 5 shows in a block circuit diagram a beam index type color CRT having a screen panel as described in connection with FIG. 4, and the related circuit. Referring to FIG. 5, the index signals from the photosensor 14 of the beam index color cathode ray tube 10 with the frequency f.sub.I equal to, for example, 8.4MHz, are supplied to a limiter 32 through as band pass filter (BPF) 31 which has the same frequency f.sub.I as the central transmission frequency. The index signals are limited at a preset level by the limiter 32 and thereby converted into nearly rectangular signals which are supplied to a PLL circuit 33. The circuit 33 has a phase comparator 34, a low pass filter (LPF) 35 and a voltage controlled oscillator (VCO) 36 in this order as viewed from the input side. The VCO 36 is set for being oscillated at a frequency 2f.sub.I at e.g. 16.8 MHz) which is double the frequency f.sub.I. The output signals from the VCO 36 are frequency divided by a 1/2 frequency divider 37 and supplied to a phase comparator 34 for phase comparison with the index signals from the limiter 32. Thus the signals from the VCO 36 of the PLL circuit 33 are phase-matched with the index signals while the frequency thereof is increased to twice index signal frequency, that is, to a value 2f.sub.I equal to, for example, 16.8 MHz. These VCO signals with the frequency equal to 2f.sub.I are supplied to a 1/3 frequency divider 39.
In the 1/3 frequency divider 39, the output signal frequency 2f.sub.I from the PLL circuit 33 is divided by 3 while the signal is formed into three color switching signals phase-shifted relative to each other by 120.degree. and having the frequency f.sub.S (=2f.sub.I /3) equal to, for example, 5.6 MHz. These three signals are then supplied to a color switching circuit 40 . To this circuit 40 are also supplied three prime color video signals, that is, red (R) signals S.sub.R, green (G) signals S.sub.G and blue (B) signals S.sub.B, via input terminals 41R, 41G and 41B, respectively. These color signals S.sub.R, S.sub.G and S.sub.B are added to one another and mixed with the aid of switches 42R, 42G and 42B of the color switching circuit 40 and thence supplied to a video output circuit 43. These switches 42R, 42G and 42B in the color switching circuit 40 are turned on and off by the three switching pulse signals having a phase shift of 120.degree. from one another. Hence, the respective color signals R, G, B are alternately outputted switching circuit 40 with the phase shift of 120.degree. to each other and within the period T.sub.S equal to 1/f.sub.S or 3/2f.sub.I which is 3/2 times the index signal period T.sub.I equal to 1/f.sub.I, these color signals being then supplied to the electron gun 13 of the cathode ray tube 10.
In this manner, two-period color switching signals are outputted each time three index stripes 25 (FIG. 4) are sensed. The color signals S.sub.R, S.sub.G and S.sub.B are sequentially switched in dependence upon these color switching signals such that the electron beam 15 sequentially irradiated on the color phosphor stripes 22R, 22G and 22B are modulated in brightness with the corresponding timing by the respective color signals S.sub.R, S.sub.G and S.sub.B, thus providing for color image reproduction.
It will be noted that, with the above described beam index color cathode ray tube, the respective widths W.sub.R, W.sub.G and W.sub.B of the red, green and blue color phosphor stripes 22R, 22G and 22B need to be different from one another for realizing so-called white balance in consideration of the difference in the light emitting characteristics of the respective phosphorescent materials. FIG. 6 shows a case wherein, for an example, only the width W.sub.G of the green (G) phosphor stripes 22G is different from the widths W.sub.R, W.sub.B of the other color phosphor stripes 22R, 22B (W.sub.G &gt;W.sub.R =W.sub.B). In a projector tube, above all, wherein a high brightness light emission is required and the respective color phosphor materials need to be excited with the large current density electron beam, it is necessary that, for maintenance of the white balance, only the width W.sub.G of the green phosphor stripes 22.sub.G be larger than the other widths W.sub.R, W.sub.G, especially in consideration that the green phosphor materials having light emitting characteristics comparable to those of the other color phosphor materials are presently not evolved.
Since the center-to-center distance between adjoining ones of the respective color phosphor stripes 22R, 22G, 22B or the pitch p in FIG. 6 is same and constant, the width W.sub.I ' of the index stripe 25 needs to be reduced if the index stripes 25 that will satisfy the requirement of the above described non-integral system concerning the number of the index stripes and that of triplets should be arranged at the central positions between adjoining ones of the color phosphor stripes 22 with a phase difference equal to p/.sub.2. Hence the index light or electro-magnetic waves resulting from impingement of the electron beam on the index stripes 25 is lowered in intensity with the result that it is occasionally infeasible to obtain normal color switching signals.
It is seen from above that, in instances wherein the widths of the respective color phosphor stripes of the beam index type color cathode ray tube are not equal to one another and the index stripes that will satisfy the above described non-integral system requirements are to be arranged with an equal pitch as described above, there results a limited allowance of the index stripe width relative to the color phosphor stripe width and hence the necessarily reduced index stripe width with reduction in the index signal intensity thus interfering with normal color switching.